Background for the invention includes U.S. patent application Ser. No. 08/972,101, filed on Nov. 17, 1997, which issued on Dec. 21, 1999, now U.S. Pat. No. 6,005,916, which is a continuation of U.S. patent application Ser. No. 08/706,205, filed on Aug. 29, 1996, which is a continuation-in-part of U.S. patent application Ser. No. 08/486,971 filed on Jun. 22, 1995, which is a continuation-in-part of U.S. patent application Ser. No. 07/961,768 filed on Oct. 14, 1992, now U.S. Pat. No. 5,588,032, all of which are incorporated herein by reference.
This invention is designed to provide improved imaging of bodies using wave field energy such as ultrasound energy, electromagnetic energy, elastic wave energy or Biot wave energy. In particular it is designed to provide improved imaging for medical diagnostic applications, for geophysical imaging applications, for environmental imaging applications, for seismic applications, for sonar applications, for radar applications and for similar applications were wave field energy is used to probe and image the interior of objects.
In particular, this invention has important applications to the field of medical diagnosis with specific value to improved breast cancer detection and screening. The present method of choice is mammography. This choice is not supported by outstanding performance of mammography, but rather because it is the most diagnostically effective for its present cost per exam. Another reason for the wide spread use of mammography is the inertia of changing to other modalities. It is known that hand-probe-based, clinical reflection ultrasound can match the performance of mammography in many cases, but only in the hands of specially trained ultrasound radiologists. Today and in the foreseeable future, there are more mammography machines and radiologists trained to use them than there are trained ultrasound radiologists that have access to high quality ultrasound scanners. Sophisticated breast cancer diagnostic centers use both mammography and ultrasound to compensate for weakness in either approach when used alone. When used alone, ultrasound and mammography require a biopsy to remove a sample of breast tissue for pathology lab analysis to determine whether a sampled lesion is cancerous or benign.
Even when mammography and ultrasound are used together the specificity for discriminating between cancer and fibrocystic condition or between cancerous tumor and a fibroadenoma is not high enough to eliminate the need for biopsy in 20 to 30 percent of lesions. Given that early diagnosis on breast cancer can insure survival and given that one woman in eight will have breast cancer in her life, it is important for the general population for cancer to be detected as early as possible. Detection on cancer in an early stage for biopsy is thus very important. However, biopsy of benign lesions is traumatic to the patient and expensive. A mammogram cost about $90 but a biopsy is about ten times more expensive. Thus it is important that a breast cancer diagnostic system have as high specificity and sensitivity to eliminate unnecessary biopsies. Increasing the rate of diagnostic true positives to near 100 percent will identify all lesions as a cancer that are cancer without the need to biopsy. But is also necessary to increase the rate of true negatives to near 100 percent to eliminate biopsy of benign lesions. Neither mammography or ultrasound or their joint use has provided the combination of sensitivity and specificity to eliminate biopsy or to detect all cancers early enough to insure long term survival after breast cancer surgery for all women that have had breast exams.
There does not seem to be any obvious improvements in present mammography or hand-probe-based, clinical reflection ultrasound that can significantly improve these statistics. However, there is reason to believe that inverse scattering ultrasound tomography or electric impedance tomography can provide improved diagnostic sensitivity or specificity when used separately or jointly with themselves and with ultrasound and/or mammography. Inverse scattering ultrasound imaging provides several advantages over present clinical reflection ultrasound that uses hand held ultrasound probes. A hand held probe is a transducer assembly that scans an area of the patient's body below the probe. Probes comprising mechanical scanning transducers and probes comprising electronically scanned arrays of transducer elements are both well developed technologies.
Inverse scattering has the following advantages over said clinical reflection ultrasound. Inverse scattering images have the following features: (1) two separate images can be made, one of speed of sound and one acoustic absorption; (2) these images are quantitative representation of actual acoustic bulk tissue properties; (3) the images are machine independent; (4) the images are operator independent; (5) the images are nearly free of speckle and other artifacts; (6) all orders of scattering tend to be used to enhance the images and do not contribute to artifacts or degrade the images; (7) the images of speed of sound and absorption create a separation of tissues into classes (fat, benign fluid filled cyst, cancer, and fibroadenoma), although the cancer and fibroadenoma values tend to overlap; (8) the speed of sound and acoustic absorption images have excellent spatial resolution of 0.3 to 0.65 mm at 5 MHz; and (9) the speed of sound images can be used to correct reflection tomography for phase aberration artifacts and improve ultrasound reflectivity spatial resolution. Inverse scattering easily discriminates fat from other tissues, while mammography and present clinical ultrasound can not.
Because of the similar values of speed of sound and acoustic absorption between cancer and fibrocystic condition (including fibroadenoma), it is not known whether inverse scattering will provide the required high lever of specificity to eliminate biopsy. Perhaps this performance could be achieved if inverse scattering were combined with reflection ultrasound or with mammography or with both.
A traditional problem with inverse scattering imaging is the long computing times required to make an image. Diffraction Tomography is a subset of inverse scattering that uses first order perturbation expansion of some wave equation. Diffraction Tomography is extremely rapid in image computation, but suffers the fatal flaw for medical imaging of producing images that are blurred and not of acceptable quality for diagnostic purposes. In out last patent application, we addressed the speed problem with inverse scattering and showed how to increase the calculation speed by two orders of magnitude over finite difference method or over integral equation methods. This speed up in performance was achieved by use of parabolic and other marching methods. This improvement in speed was sufficient to allow single slices to be collected at frequencies of 2 MHz using 3-D scattered data collected on a 2-D detector, but making a full 3-D image at once or from stacked 2-D slices would have required computing speed not available then and even mostly now.
Another imaging modality that has been investigated to detecting breast cancer is EIT (electrical impedance tomography). This modality has been investigated for many years and many of its features are well known. One of its great advantages for breast cancer detection is its high selectivity between cancer and fibrocystic conditions including fibroadenoma. Cancer has high electrical conductivity (low electrical impedance) while fibrocystic conditions and fibroadenoma have low electrical conductivity (high electrical impedance). However, EIT has poor spatial resolution. Also EIT requires the use of inversion algorithms similar (in a general sense) to those of inverse scattering. In addition EIT algorithms have mostly been used to make 2-D images. Work on making 3-D EIT images is less developed because of the increased computer run time of 3-D algorithms.
Other problems with mammography may be listed, such as the pain associated with compressing the breast between two plates to reduce the thickness of the breast in order to improve image contrast and cancer detection potential. Another problem is accumulated ionizing radiation dose over years of mammography. It is true that a single mammogram has very low x-ray dose, especially with modem equipment. However, if mammograms are taken every year from age 40 or earlier, then by age 60 to 70 the accumulated dose begins to cause breast cancer. The effect is slight, and the benefits of diagnosis outweigh this risk. Nevertheless, it would be and advantage to eliminate this effect. Mammography is not useful in younger women (because of the greater density of their breasts) or older women with dense breasts (such as lactating women). About 15 percent of women have breasts so dense that mammography has no value and another 15 percent of women have breasts dense enough that the value of mammography is questionable. Thus 30 percent of all mammograms are of questionable or zero value because of image artifacts from higher than normal beast density.